True time delay generation utilizing broadband light source with fiber chirp grating array and acousto-optic beam steering and 2-D architectures

ABSTRACT

System and method for rapidly reconfigurable two-dimensional true time delay generation for phased array antennas is described. The system utilizes a broadband light source, an array of fiber chirp gratings in a single fiber, and an acousto-optic spectrometer to generate a time-delayed linear grating. The grating is subsequently rotated to the desired angle utilizing an acousto-optic device having no moving parts.

RELATED U.S. PROVISIONAL PATENT APPLICATION

This application is related to U.S. Provisional Patent Application No.60/036,408, pending, entitled “A 2-D True Time Delay Architecture forPhased Array Antennas Using Fiber Chip Gratings and Acousto-Optic BeamDeflectors” by Eung G. Paek filed Jan. 31, 1997, makes claim to theinvention disclosed therein, and is a continuation-in-part of said U.S.Provisional Patent Application.

FIELD OF THE INVENTION

This invention relates to phased array antenna systems, and, moreparticularly, relates to optical true time delay generation methods andarchitecture for such systems.

BACKGROUND OF THE INVENTION

The phased array antenna is one of the most advanced radar technologieswhich allows multiple bean pointing and fast non-mechanical steering ofmicrowave beams. The technology has promise for broad-band (2-20 GHz)free-space radar communications that can be used for a variety ofcommercial and military applications. Beam pointing/steering controlsystems are known, including true time delay systems and phase shiftsystems, for phased array antenna, true time delay systems beingpreferable since the steered beam angle is independent of frequency andsquint is eliminated.

For a given microwave frequency f and number of microwave radiatingelements along one direction N, the maximum time delay Δt_(max) requiredto steer a beam over ±90° is given by N/f. Also, the minimum temporalresolution Δτ_(min) to achieve resolution R is given by 1/(f·R).Assuming a frequency range of 2-20 GHz, N=100 and R=1,000, Δt_(max)=5-50nsec and Δτ_(min)=0.05-0.5 psec.

In conventional electronic RF systems, true time delay is achieved usingswitched lengths of electrical waveguide or cable. Such devices tend tobe bulky, expensive, have high loss at high frequencies, and aresusceptible to electrical crosstalk (due to electromagneticinterference) and temperature induced time delay changes. Recentadvances in photonic technology can provide a better implementation oftrue time delay due to a natural high parallelism and large bandwidth aswell as immunity to electromagnetic interference.

Heretofore known or suggested photonic true time delay systems have beenconfigured so that each microwave element requires R fixed time delaygenerators, R switches and an R to 1 combiner. Thus, for a twodimensional (2-D) array with N² elements in such systems, N²R timedelays and N²R switches have been required. The insertion loss is mainlydetermined by the R to 1 combiner and is given by 10 log₁₀R. Althoughsuch a system is capable of adaptive beam forming as well as beamsteering, it requires a tremendous amount of complexity, making itshardware implementation extremely difficult.

Although this complexity can be reduced to some degree by free-spacepath-switching methods, this still requires a cascaded array of manyindependent time-delay generators and parallel (N²) switches in 2-Dspatial light modulators. Moreover, thus configured, the system presentsother limitations, such as speed and path-dependent insertion loss.

A highly dispersive fiber prism method has been suggested and/orutilized that can significantly reduce the complexity as describedabove. However, this method requires very long (20 km for 1 GHz), N²fiber bundles and a fast tunable narrow linewidth light source withbroad tuning range. It has been suggested that the long length could besignificantly reduced by using an array of fiber gratings, butsignificant problems with this implementation would yet be posed. Mostof the heretofore suggested approaches for use of fiber gratings as ameans to generate true time delays employ an array of normal singlefrequency fiber gratings, the desired time delays being selected by atunable narrow linewidth light source. To achieve high resolution, botha broad tuning range and a narrow linewidth are required. Moreover, thewavelength would need to be changeable rapidly (within a fewmicroseconds—a speed unattainable by current laser technology) foreffective implementation.

In addition, two dimensional (2-D) extension architecture for suchphotonic true time delay systems as have been heretofore suggested couldutilize further improvements. Conventional image rotation has beenaccomplished, for example, by rotating a dove prism by an angle θ aroundthe optical axis, the output image thus being rotated by 2θ. Suchconventional rotation thus requires mechanical movement of componentsand is, therefore, inherently slow and lacking adequate unreliability.

SUMMARY OF THE INVENTION

This invention provides a true time generating system and method forboth one dimensional and two dimensional generation of time delayedgratings for use with phased array antenna systems. This inventionincludes a delay encoder operable with a broadband light source andutilizing a single optical fiber having an array of fiber chirp gratingstherein, an optical signal decoding device for receiving a wavelengthencoded light signal and providing as an output therefrom a time delayedgrating, and image rotation utilizing acousto-optics and without movingmechanical parts.

The delay encoder fiber chirp gratings are configured so that differentwavelengths of the light from the light source are reflected at uniquelocations at each individual fiber chirp grating, the locationscorresponding to different selected time delays, thereby providing awavelength encoded light signal output. The decoding device receives thewavelength encoded light signal output and utilizes this output toprovide a time delayed linear grating as an output therefrom.

An optical amplifier amplifies the wavelength encoded light signal andan acousto-optic deflector having a variable acoustic signal input ispositioned to receive the amplified light and disperse the light atselected diffraction angles variable by an acoustic signal at the input.A window is positioned at the output plane from the deflector forselection of an output spectrum from the dispersed light, spectrumselection controlled by diffraction angle selection at the acousto-opticdeflector.

The method for generating time delays for phased array antennas of thisinvention includes launching broadband light into an optical fiberhaving a plurality of selectively located fiber chirp gratingstherealong to provide a wavelength encoded light signal output anddispersing the wavelength encoded light signal output at an outputplane. A spectrum from light dispersed at the output plane correspondingto a selected one of the fiber chirp gratings at the optical fiber isselected to thereby provide a selected time delayed linear grating, thegrating being rotated to a desired output angle.

Utilizing this invention, the complexity of heretofore known systems canbe significantly reduced, requiring only a number of fiber chirpgratings in a single fiber providing the number of different time delaysdesired. The compact system allows a broad range of time delays that canbe reconfigured within a few microseconds, and because of fewer or nomechanical elements and switches, is inherently more reliable.

It is therefore an object of this invention to provide true time delaygeneration systems and method utilizing a broadband light source and afiber chirp grating array in a single fiber.

It is another object of this invention to provide true time delaygeneration systems and method including fully optical delay selectionand acousto-optic image rotation.

It is another object of this invention to provide optical true timedelay generation systems of reduced complexity, requiring only a numberof fiber chirp gratings in a single fiber providing the number ofdifferent time delays desired.

It is still another object of this invention to provide true time delaygeneration systems for phased array antenna systems which are compact,allow a broad range of time delays that can be reconfigured within a fewmicroseconds, and are highly reliable.

It is still another object of this invention to provide a true timedelay generating system including a broadband light source, delayencoding means including an optical fiber for receiving light from thebroadband light source, the optical fiber having at least a firstselectively located fiber chirp grating defined therein so thatdifferent wavelengths of the light from the light source are reflectedat unique locations at the fiber chirp grating, the locationscorresponding to different selected time delays, thereby providing awavelength encoded light signal output, and decoding means for receivingthe wavelength encoded light signal output and utilizing the wavelengthencoded light signal output to provide a time delayed linear grating asan output therefrom.

It is yet another object of this invention to provide a 2-d true timedelay generating system for phased array antennas including, an opticalfiber for receiving light from a broadband light source and including anarray of fiber chirp gratings defined therealong, different wavelengthsof light from the light source reflected at unique locations at thefiber chirp gratings, the locations corresponding to different selectedtime delays, an acousto-optic deflector having a variable acousticsignal input for receiving light reflected at the fiber chirp gratingsof the optical fiber and dispersing the reflected light at selecteddiffraction angles variable by an acoustic signal at the input, and anacousto-optic image rotator having no moving parts for receiving lightdispersed at the acousto-optic deflector and rotating the dispersedlight to a selected output angle.

It is still another object of this invention to provide an opticalsignal decoding device for receiving a wavelength encoded light signaland providing as an output therefrom a time delayed grating forutilization in a phased array antenna system, the device includingamplifying means for optically amplifying a wavelength encoded lightsignal, an acousto-optic deflector for receiving the amplified lightsignal and dispersing the light signal at selected diffraction angles toan output plane, and a window at the output plane for selection of anoutput spectrum from the dispersed light signal, spectrum selectioncontrolled by diffraction angle selection at the acousto-opticdeflector.

It is yet another object of this invention to provide a method forgenerating time delays for phased array antennas including the steps oflaunching broadband light into an optical fiber having a plurality ofselectively located fiber chirp gratings therealong to provide awavelength encoded light signal output, dispersing the wavelengthencoded light signal output at an output plane, selecting a spectrumfrom light dispersed at the output plane corresponding to a selected oneof the fiber chirp gratings at the optical fiber to thereby provide aselected time delayed linear grating, and rotating the time delayedlinear grating to a desired output angle.

With these and other objects in view, which will become apparent to oneskilled in the art as the description proceeds, this invention residesin the novel construction, combination, arrangement of parts and methodsubstantially as hereinafter described, and more particularly defined bythe appended claims, it being understood that changes in the preciseembodiment of the herein disclosed invention are meant to be included ascome within the scope of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate a complete embodiment of theinvention according to the best mode so far devised for the practicalapplication of the principles thereof, and in which:

FIG. 1 is a schematic illustration of a 2-d true time delay generationsystem architecture in accord with this invention;

FIG. 2 is a chart illustrating fiber grating position and Braggwavelength correlation;

FIG. 3 is a functional illustration of the time delayed linear gratinggeneration system of this invention including encoding and decoding;

FIG. 4 is another functional illustration of the system substantially inaccord with FIG. 3

FIG. 5 is a chart illustrating Bragg wavelength distribution along thefiber with reference its FIGS. 2 and 3;

FIG. 6 is a functional illustration of acousto-optic image rotationarchitecture in accord with this invention; and

FIG. 7 is a functional illustration of another embodiment of the systemof this invention.

DESCRIPTION OF THE INVENTION

True time delay control architecture 13 of this invention for generationof a one dimensional (1-D) diffraction grating light pattern whoseperiod (temporal time delay in the true time delay case) and orientation(2-D extension) can be changed rapidly is schematically illustrated inFIG. 1. As may be appreciated from FIG. 1, the complexity of this truetime delay system is significantly reduced compared with conventionalsystems in which provision for all the possible delays are required foreach microwave radiating element of a phased array antenna system.

In FIG. 1, RF input signal 15 is modulated by CW light source 17 atelectro-optic modulator 18 providing light having a broad spectrallinewidth (for example, 50 nm), and is launched into single opticalfiber 19 having an array of fiber chirp gratings 21 defined therealong(see FIG. 3). Formation of such gratings in optical fiber may be doneusing known techniques. Each fiber chirp grating 21 defined in fiber 19has a unique chirp ratio, and thus defines unique time delays. The lightfrom each fiber chirp grating is dispersed along the x direction, oraxis, by acousto-optic beam deflector 23 and is stretched along the ydirection, or axis.

By adjusting the frequency of acoustic signal applied to acousto-opticbeam deflector 23 (for example, using an rf signal adjustable in a rangefrom 30 MHz to 60 MHz under program control to an rf generator orfrequency synthesizer), the spectrum from the desired fiber chirpgrating 21 is selected and positioned on output window 25. Thetime-delayed grating thus obtained is subsequently rotated to thedesired angle by ultrafast image rotator 27 before output at N×N lightto microwave converters 29. In the following, a more detailedexplanation the 1-D true time delay system of this invention and itsextension to a 2-D case in accord with this invention is provided.

In the system of this invention, an array of equally spaced N timedelays is generated by a single fiber chirp grating 21. As shown in FIG.2, the Bragg wavelength of a fiber chirp grating varies linearly as afunction of position, from λ_(S) to λ_(L) with the center wavelengthλ_(C), over the length Δl. The wavelength chirp ratio Γ=Δλ/Δl,(Δλ=λ_(L)−λ_(S)), is kept constant within a fiber chirp grating. Theresultant relative time delays arising from various portions (i.e.,single frequency fiber Bragg gratings) of the fiber chirp gratingdistribute uniformly ranging from 0 to the maximum value Δt=2·η·Δl/C,where η is the refractive index of the fiber core and C is the speed oflight in vacuum. Each of these time delays is encoded by thecorresponding wavelength, while keeping the ratio Δt/Δλ constant.

One should also note that fiber chirp gratings can be superposed toreduce the total fiber length owing to the phase matching selectionproperty of a volume grating, as long as the total refractive indexchange does not exceed the maximum limit of the fiber. When the fiberchirp grating is long, it can be discretized to form an array of N shortsinusoidal gratings.

The 1-D true time delay control system of this invention includes fiberchirp grating encoder architecture 31 and acousto-optic spectrometerdecoder architecture 33 as shown in FIGS. 3 and 4 (each illustratingdifferent aspects of and/or alternatives for the implementation of thisinvention). Both encoding and decoding are achieved by wavelengths.Fiber chirp grating encoder 31 includes a light pulse generator (either17/18 as shown in FIG. 1 or utilizing another broadband source such asLED 35 (FIG. 4), or an amplified spontaneous emission source, modulatedby an external electro-optic modulator) with broad spectral bandwidthand single fiber 19 (terminated at one end) with an array of fiber chirpgratings 21 in it. Light is circulated at circulator 37 in aconventional fashion (see FIG. 4).

FIG. 5 (referring to FIGS. 2 and 3) illustrates Bragg wavelengthdistribution inside fiber 19 as a function of position. The i th fiberchirp grating 21 has the length Δl(i)=Δl(1)·i, and all fiber chirpgratings 21 have the same amount of wavelength bandwidth,Δλ(i)=Δλ_(FCG)=constant. Therefore, the wavelength chirp ratio of the ith fiber chirp grating 21, Γ(i), is given by the relation Γ(i)=Γ(1)/i,(or Δt(i)=i·Δt(1), where Γ(1) and Δt(1) are defined for the first fiberchirp grating 21.

If light from pulsed source 35 (or, alternatively, CW source 17, or thelike, modulated by electro-optic modulator 18 using pulsed microwavesignal 15 as shown in FIG. 1) with a wide spectral bandwidth, Δλ_(SC),is launched into fiber 19, each particular wavelength of the input lightis reflected at a corresponding unique fiber chirp grating 21 locationto give the desired time delay. In this way, each time delay is encodedby the corresponding wavelength.

The wavelength encoded light signal output from circulator 37 isoptically amplified in a single fiber channel by erbium doped fiberamplifier 39 (or an equivalent means of amplification), and is connectedwith free space acousto-optic spectrometer decoder 33 includingspherical lenses 41 and 43 and acousto-optic beam deflector 23.Acousto-optic spectrometer decoder 33 disperses the incoming light likea normal prism, the primary difference between acousto-opticspectrometer decoder 33 and a normal prism being that the diffractionangle can be rapidly (within a few microseconds) varied by simplychanging the acoustic frequency applied to acousto-optic beam deflector23. In acousto-optic spectrometer decoder 33, the light with wavelengthλ is focused to a point separated from the optical axis byx=λ·f_(AO)·f_(L)/V_(AO), where f_(AO), f_(L) and V_(AO) representacoustic frequency applied to acousto-optic beam deflector 23, focallength of lense 41 and acoustic velocity inside the acoustic medium (forexample, focal length of lenses 41 and 43 are typically about 5 cm and20 cm, respectively, and acoustic velocity is typically about 600 m/s inthe medium). The spatial extent of the spectrum generated by each fiberchirp grating 21 is given by Δx=Δλ·f_(AO)·f_(L)/V_(AO).

The temporal delay over each spectrum is Δt(i)=2·η·Δl(i)/C. Since Δt∝Δλand Δx∝Δλ, it follows that Δx∝Δt. In other words, time delay isuniformly distributed along x providing a suitably time delayed lineargrating. At the output plane, window 25 is placed to select the spectrumfrom the desired fiber chirp grating only. By varying the acousticfrequency f_(AO) such that f_(AO)(i)·λ_(C)(i)=constant, the desired i thspectrum can be centered at output window 25 (other spectra beingcentered at the window by adjusting the frequency accordingly; see FIG.4).

Also, multiple beam forming can be easily achieved by applying manyacoustic sinusoidal frequencies simultaneously to acousto-optic beamdeflector 23, without the need for any hardware changes. Window 25 ischaracterized by a slit with an aperture size of about 0.5 mm to 5 mmdepending upon the number of radiating elements.

In this way, time delays encoded to wavelengths in encoder 31 aredecoded back to time delays. Owing to fiber chirp gratings 21 andacousto-optic spectrometer decoder 23 combined with output window 25, Nelement encoding/decoding is achieved simultaneously in parallel in asimple and compact free space system. One great advantage of this freespace spreading over the heretofore known dispersive fiber method isthat it can avoid the complicated N² long fiber bundles. In addition,adequate room for optical amplification or means for compensation formisalignment due to temperature changes remains without making thesystem overly cumbersome. The compact 1-D decoder 33 of this inventioncan also be integrated using integrated waveguide optics.

The components utilized above may be any of those known to skilledpractitioners in the art. For example, modulator 18 may be aMach-Zehnder electro-optic modulator or multiple-quantum-well basedelectro-absorptive modulator, and fiber 19 may be formed from opticalfiber having germania-doping to increase light sensitivity. Circulator37 may be a three port device using Faraday isolators to allow reversesignal entering one of the two output ports to be transmitted to theother output port as a usable signal while being completely isolatedfrom the input signal, fiber amplifier 39 is an Erbium-doped amplifier,and lenses 43 are preferably spherical lenses with doublet elements toreduce aberrations. Acousto-optic beam deflector 23 is preferablycharacterized by large time-bandwidth product (defined by aperture timemultiplied by the frequency bandwidth; more than 1,000) and highdiffraction efficiency (for instance, utilizing slow shear-mode TeO₂material; more than 30% efficiency preferred at the 1.55 micronwavelength region.

The above-described 1-D true time delay system could be extended to the2-D case by cascading as heretofore known. In such case, advantagesremain in that only N+1 fibers, as opposed to N² fibers utilized byprevious systems, would be required. Moreover, the free spaceacousto-optic spectrometer 33 can be shared Among N elevation elements(typically up to at least about 1,000).

However, a better means in accord with this invention for extending to2-D true time delay, and which significantly reduces complexity becauseof elimination of moving parts, provides ultrafast image rotationutilizing acousto-optic image rotator 27 (a dove prism-like arrangement)as shown in FIG. 6. The system includes a pair of xy-acousto-optic beamdeflectors 55 and 57 and circular cylindrical mirror 59 to generate therequired inversion operation.

In principle, the rotating wedge prism portions of a conventional doveprism have been replaced by xy-acousto-optic beam deflectors 55 and 57,each consisting of a pair of acousto-optic beam deflectors 61/63 and65/67, respectively, sandwiched together with transducers alongorthogonal directions (slow shear mode TeO₂ crystals could be utilized;however, a more compact (10 mm×10 mm×2 mm) crossed deflector with bothtransducers on the same crystal is preferable). By adjusting thefrequencies of acoustic signals applied to the acousto-optic beamdeflector pairs 61/63 and 65/67, beam direction can be changed alongarbitrary directions, just as a wedge prism does. Since circularcylindrical mirror 59 xs rotationally symmetric, its rotation is notrequired. However, to prevent unwanted mirror distortion owing to thecurvature of the circular mirror surface the cylindrical mirror ispreferably discretized to multiple facets. Also, incoming light isfocused to have a minimum size on the mirror surface. Therefore, bysimply varying the acoustic frequencies, an image can be rotated toarbitrary selected angles, without any moving parts while yet preservingoptical transparency.

The rotation angle of an image output from window 25 can be reconfigured(for example, rotated by 180°) within a few microseconds in aprogrammable manner utilizing frequency synthesizers 69 and 71 (forexample, a frequency synthesizer with a frequency sweep range of 30 MHzto 70 MHz, with frequency accuracy of less than 1 Hz and drive outputpower of up to 1 watt, interfaced with a PC for with fast parallelconnectors to allow fast access time, preferably on the order ofmicroseconds). To prevent the unwanted distortion owing to the curvatureof the circular mirror surface, the cylindrical mirror is discretized tomultiple facets. Also, incoming light is focused to have a minimum beamsize on the mirror surface by using lens 73. Lens 75 is used to form animage at output plane 83 (for example, lenses 73 and 75 are sphericaldoublet lenses having a 20 cm focal length and opposite orientation tocompensate for the aberration with each other). Special lens designscould, alternatively, be conceived to compensate for the fixeddistortion due to the circular cylindrical mirror surface. It should berecognized that lenses 73 and 75 as shown in the FIGURE could bealternatively positioned relative to beam deflectors 55 and 57,respectively, with each positioned adjacent to opposite ends of mirror59.

In operation, light representing the time delayed linear grating islocated at the front focal plane of lens 73, and is deflected by thecrossed acousto-optic beam deflector pair 55 to a selected output angleunder control of the acoustic signal input. At the deflected angle, thelight is focused on the surface of cylindrical mirror 59. Afterreflection thereat, the light is deflected again by crossedacousto-optic beam deflector pair 57 (again under control from programcontrolled frequency synthesizer 71 output acoustic signal) forming aselectively rotated image at output plane 83 (to N×N light to microwaveconverters 29 of FIG. 1.

The system described hereinabove provides significant advantage over nowknown conventional systems. For example, conventional systems, includinghighly dispersive fiber delay systems, require significantly more timedelay generators switches and/or fiber bundles (and/or significantlygreater fiber length, i.e., translating into much greater total fibervolume) than is required for this invention. These advantages aresummarized in Table 1.

TABLE 1 Proposed Conventional HD Fiber FCG/AOBD (1) (2) (3) # Delays N²RN² NR # Switches N²R 2 2 # Bundles N² N² 1 # Splitters (1 to N) 2N² N +1 0 100 nsec length 20 m 20 Km 10 m Total fiber volume 3 m × 3 m × 0.1 ×0.1 × 0.001 ××0.001 × 10 = 20 m = 20,000 = 10⁻⁵ m³ 180 m³ (4) 200 m³Complexity of various systems: (1) Conventional Photonic Systems, (2)Highly Dispersive fiber systems, and (3) this invention; where N =elements along one direction and R = resolution (Note: 10 cm × 10 cm ×1000 is approximated to 3 m × 3 m)

The maximum number of wavelength channels (or time delays) in thearchitecture of this invention is determined by such factors as thepassband of fiber amplifier 39 (typically 50 nm), the bandwidth of anFBG (i.e., fiber Bragg grating, for example, typically 0.04 nm for a 10cm long FBG) and the resolution of acousto-optic spectrometer 33.Normally, more than 1,000 time delays can be generated utilizing theinvention as illustrated heretofore.

For dense systems, requiring significantly more than 1,000 differenttime delays (for example up to at least about 10,000), multiplexingarchitecture 85 as shown in FIG. 7 may be employed, for example, using roptical switches 87 and r circulators 89 in conjunction with r fibers 19having fiber chirp gratings 21 therein (r equalling the selectedextensions required to achieve the desired number of time delays). Inthis case, fiber chirp gratings 21 are distributed in several fibers andthe desired fiber is selected by the corresponding optical switch 87(for example, under program control from a PC). Even for the systemshown in FIG. 7, the number of switches required is determined by themultiplexing number (typically 10) instead of Nr (typically 1,000) toN²R (typically 10⁷) as would heretofore have been required inconventional systems. An alternative way of multiplexing using opticalswitches is to employ the conventional binary fiber optical delay lineconcept by cascading the fibers in series.

As may be appreciated from the foregoing, true time delay controlutilizing a specially designed array of fiber chirp gratings in a singlefiber with a broadband light source input, and that requires nointermediate photon-to-electron conversion processes, is provided forphased array antenna systems. Each fiber chirp grating in the array isdesigned to provide a unique linear chirp ratio, and wavelengths do notoverlap. Generation of time delays from wavelengths (i.e., utilizing thedecoder of this invention) is rapidly accomplished along a wide tuningrange and provides a tailored array of selected linear chirp time delaysin parallel. 2-D extension is accomplished with far fewer elements thanheretofore known and with no moving parts.

What is claimed is:
 1. A 2-d true time delay generating system forphased array antennas comprising: an optical fiber for receiving lightfrom a broadband light source and including an array of fiber chirpgratings defined therealong, different wavelengths of light from saidlight source reflected at unique locations at said fiber chirp gratings,said locations corresponding to different selected time delays; anacousto-optic deflector having a variable acoustic signal input forreceiving light reflected at said fiber chirp gratings of said opticalfiber and dispersing said reflected light at selected diffraction anglesvariable by an acoustic signal at said input; and an acousto-optic imagerotator having no moving parts for receiving light dispersed at saidacousto-optic deflector and rotating said dispersed light to a selectedoutput angle.
 2. The 2-d true time delay generating system of claim 1further comprising a window at an output plane from said acousto-opticdeflector for selection of a selected spectrum of said dispersed light.3. The 2-d true time delay generating system of claim 1 wherein saidfiber chirp gratings in said array are sufficient in number so that upto at least about 1000 time delays corresponding to wavelengthreflection locations are provided.
 4. The 2-d true time delay generatingsystem of claim 1 further comprising a plurality of second opticalfibers each having an array of fiber chirp gratings defined therealong,and multiplexing means associated with said optical fibers so that up toat least about 10,000 time delays corresponding to wavelength reflectionlocations are provided.
 5. The 2-d true time delay generating system ofclaim 1 wherein said acousto-optic image rotator includes first andsecond crossed acousto-optic deflectors at opposite ends of acylindrical mirror.
 6. An optical signal decoding device for receiving awavelength encoded light signal and providing as an output therefrom atime delayed grating for utilization in a phased array antenna system,said device comprising: amplifying means for optically amplifying awavelength encoded light signal; an acousto-optic deflector forreceiving the amplified light signal and dispersing the light signal atselected diffraction angles to provide multiple wavelength spectralinearly arrayed at an output plane; and a window at said output planefor selection of a selected output wavelength spectrum from saidmultiple wavelength spectra linearly arrayed at said output plane,wherein said selection of a selected output wavelength spectrum iscontrolled by diffraction angle selection at said acousto-opticdeflector.
 7. The optical signal decoding device of claim 6 wherein saidacousto-optic deflector includes input means for a selectively variableacoustic signal to effect diffraction angle changes within a fewmicroseconds.
 8. The optical signal decoding device of claim 6 furthercomprising an acousto-optic image rotator with no moving parts forrotating said output spectrum to a selected output angle.
 9. The opticalsignal decoding device of claim 8 wherein said acousto-optic imagerotator includes a first set of crossed acousto-optic deflectors fordeflecting incident light at selected angles.
 10. The optical signaldecoding device of claim 8 wherein said acousto-optic image rotatorincludes a cylindrical mirror for rotating incident images at selectedangles to an output.
 11. A method for generating time delays for phasedarray antennas comprising the steps of: launching broadband light intoan optical fiber having a plurality of selectively located fiber chirpgratings therealong to provide a wavelength encoded light signal output;dispersing said wavelength encoded light signal output at an outputplane; selecting a spectrum from light disperser at said output planecorresponding to a selected one of the fiber chirp gratings at saidoptical fiber to thereby provide a selected time delayed linear grating;and rotating said time delayed linear grating to a desired output angle.12. The method for generating time delays of claim 11 further comprisingthe steps of serially selecting different spectra from light dispersedat said output plane corresponding to selected different fiber chirpgratings to selectively provide different time delayed linear gratings,and rotating said different gratings to desired output angles.
 13. Themethod for generating time delays of claim 11 wherein the step ofselecting a spectrum from light dispersed at said output plane includesthe step of selectively periodically changing diffraction angle ofdispersion of said wavelength encoded light signal output to seriallypresent selected spectra at an output window at said output plane. 14.The method for generating time delays of claim 11 further comprising thesteps of launching broadband light into a plurality of second opticalfibers each having an array of fiber chirp gratings defined therealongso that up to at least about 10,000 time delays are provided.
 15. Themethod for generating time delays of claim 11 further comprising thestep of configuring said fiber chirp gratings so that wavelength chirpratio is constant within any one of said fiber chirp gratings, with eachdifferent said fiber chirp grating having a unique wavelength chirpratio.
 16. The method for generating time delays of claim 11 furtherwherein the step rotating said time delayed linear grating includesacousto-optically rotating said grating without mechanical movement ofparts.